Objective - The proposed study is designed to investigate human spinal neural circuit plasticity associated with locomotor training following motor incomplete spinal cord injury (SCI) and determine the relationship between that plasticity and the extent of functional recovery of gait. Research Plan - In the proposed study we will study subjects with sub-acute motor incomplete SCI before, multiple times during, and 3 months following a course of robotic locomotor training. We will assess spinal reflex circuit functioning as measured by H reflex testing and compare that with functional recovery of over ground gait. We have studied H-reflexes in the soleus muscle during stepping before and after 3 months of locomotor training and found that those reflexes decrease or normalize in patients who recover relatively greater over ground gait speeds but that those reflexes actually increase in patients who only recover slower over ground gait speeds. We are interested in determining the time course of reflex changes in these patients and in finding out if patients who recover faster gait speeds and more normal reflexes first experience a relative increase in hypereflexia when they have only achieved slower gait speeds. This would indicate we should train those with slower recovered gait speeds for longer periods of time. We also propose to study H-reflexes simultaneously in other leg muscles to determine if the soleus H-reflex recovery pattern is common to all lower limb muscles or not. Methods - Once patients meet inclusion/exclusion criteria and provide informed consent, they will be fitted for and oriented to the robotic gait orthosis, the Lokomat, and locomotor training in that device. Before, monthly during and 3 months after 4 months of locomotor training, patients will be assessed electrophysiologically and by clinical measures of gait recovery. Functional gait measures will be speed, measured during a 10 meter walk, and stepping measurements using the GAITRite system to document parameters like stride length and stance width. Electrophysiological testing will be carried out with a new technique to generate simultaneous H reflexes in multiple lower limb muscles. Stimulation of the lumbar dorsal roots generates these dorsal root motor potentials in all leg muscles and can be studied under various conditions at the same time points during locomotor training as gait measures are taken. Specifically, we will study dorsal root motor potentials at two different treadmill speeds and at two different body weight supports in the Lokomat. Once baseline electrophysiological and functional gait measures have been taken, patients will begin 16 weeks of locomotor training in the Lokomat. Over time, the locomotor training speed will be increased and the BWS decreased as can be tolerated by the patient. Once training has begun, functional gait measurement and electrophysiological testing will occur every 4 weeks, staggered by two weeks. That is to say that at 2, 6, 10 and 14 weeks after training begins functional gait measures will be repeated and at 4, 8, 12 and 16 weeks, electrophysiological measures will be taken. Once patients have completed their locomotor training, they will be asked to return 3 months later for follow up functional gait measures and electrophysiological testing to determine whether those have changed over time since training. PUBLIC HEALTH RELEVANCE: Spinal cord injury (SCI) occurs in veterans and requires that the VA medical system delivers the best possible care for these veterans. Locomotor dysfunction can be improved with locomotor therapy in veterans with motor incomplete SCI but we do not know how the nervous system is changing with this training or whether we could get better training effects and better recovery if we better understood the underlying neural plasticity that gives rise to that recovery. Hopefully, learning more about the neurophysiological mechanisms by which locomotor training improves gait recovery in motor incomplete SCI will present new ideas about how to improve upon that treatment or offer treatment to patients that currently do not benefit from locomotor training after SCI.